From eyewitness tweets to Facebook calls for cleanup, how technology provides context in a crisis

BALTIMORE—The first I heard of Monday’s riots here was on Facebook. In a parent group usually populated by requests for play dates, a mom posted that violence had broken out not less than a mile from my home. After days of peaceful protests, I was at first incredulous. I looked out my window at the sedate cafe across the street: Nothing out of the ordinary.

But there on Twitter, my feed was lighting up from the moment the first rocks...